Mothers of an infant are much less likely to exercise regularly compared with women who have older children or no children. This low level of physical activity (PA) contributes to postpartum weight retention which can predict levels of obesity up to 15 years later. Ethnic minorities have higher rates of inactivity and gain more weight following childbirth, which places them at increased risk for cancer and other chronic diseases consistent with known health disparities for these ethnic groups. Few PA interventions have been designed to increase PA in women with an infant, especially ethnic minority women. This study will test the efficacy of a tailored intervention to increase and maintain PA in a multiethnic population of 384 young, healthy postpartum women living in Hawaii. Women will be recruited from health care settlings, mother/baby support groups, and through a coalition of over 800 community organizations that provide clinical care and promote maternal and child health in women, many of whom are low-income. Subjects will be randomly assigned to either a tailored postpartum counseling intervention on PA or standard care for PA. The tailored PA intervention will address key personal, social, and environmental factors derived from Social Cognitive Theory (SCT) and the Transtheoretical Model (TTM). Also, multimodal contacts (telephone, e-mail, website) will be used to deliver theoretically-derived, culturally sensitive PA counseling,behavioral skills training (e.g., goal setting), and local resources/referrals tailored to a mother with an infant. The standard care condition receives AHA/ACSM print materials/e-mails and referral to PA internet resources. The number of contacts will be similar for both conditions.The primary PA outcome is minutes of moderate/vigorous physical activity (MVPA) per week, as measured by the International Physical Activity Questionnaire, with validation by accelerometers worn by all subjects. Exercise tolerance, measured via the 6-minute walk test, is a secondary outcome. Condition differences in the initiation of PA will be tested at 6 and 12 months post- baseline, with maintenance of MVPA evaluated 24-months post-baseline. Key psychosocial, physiological, and cultural factors will be tested as mediators or moderators of PA, for example: self-efficacy, social support, TTM processes of change, ethnicity, BMI, and cultural values. This study will serve as a model for the design and implementation of PA interventions for at-risk ethnic minority postpartum women.